Activity study of the Suminoe flexure, a branch of the Uemachi fault system and estimated sea level change during the past 10, 000 yrs, in the western Osaka Plain, central Japan

Abstract

Borehole and seismic-reflection data provide no evidence for recent activity of the Suminoe flexure, a branch of the Uemachi fault system near Osaka since 14, 000 ¹⁴CyBP. Using cores from six boreholes, we studied the sedimentology, paleontology, micropaleontology, tephrochronology, and radiocarbon age of late Quaternary deposits on the flexure. We also made an S-wave seismic reflection survey to check the architecture of these deposits. The architecture of these units shows that the average vertical slip rate on the flexure has not exceeded 0.2 m/ky, and that the most recent faulting predates 14, 000 ¹⁴CyBP. The Holocene deposits further shows that relative sea level was about -19 m at 8, 500 ¹⁴CyBP, and that it rose rapidly between 8, 500 and 6, 000 ¹⁴CyBP.